TOPIC:

For my Porsche 959 restore, I'm looking for 'Parts C'

Weld it up using the soldering iron method?

That's a thought... haven't soldered plastics before.

Its not that hard, I've repaired a hornet chassis this way, just behind the front upper suspension arm. Just remember not to sniff the fumes. You can also practice on sprue to get a feel for how fast it melts and flows, and also use extra sprue to fill gaps or melt into large cracks, a bit like solder.

Don't get the plastic too hot though, as it will make it brittle - Use a clean soldering iron tip, a low wattage soldering iron (15w is best, you can get away with 25w if you're careful), heat the plastic JUST enough to melt it. If you heat it too much, it'll start to look "wet", which is bad, a sign that the "plasticiser" (The stuff that makes the plastic not-brittle) is being forced out.

Although this part's probably too small for my normal method, what I usually do is to run the soldering iron tip edge-on along the crack to form a "root weld", then use the flat of the tip to fold molten plastic into the groove from either side, or like Yogi said, use sprue of the same material as "filler rod".
Be aware that you can only join plastics of the exactly same type, so doing things like fixing polyethylene Holiday Buggy bodies with bits of ABS sprue just aint gonna work.... To check for material type, find the recycling mark.

Safety tip - don't breath the fumes. Don't touch the molten plastic (Cos it will stick to you & burn) or anything that looks burnt/sooty - some plastics give off some pretty nasty chemicals when heated (eg see COSHH on Hydroflouric acid).